Best Possible Pretreatment

Q. If you could choose only one method of pretreatment, chemical (or chemicals) of your choice or mechanical (sandblasting), for powder coating your personal set of brand new, bare, rust-free steel patio furniture, which would it be?—J.D.

A. Mechanical cleaning (abrasion) is a common way to prepare a part for coating. It has a great advantage over chemical cleaning because it promotes exceptional physical adhesion. The coating flows over a much larger surface area and into all the roughened areas created by the abrasion which holds it on tightly, providing the necessary adhesion—the first challenge to getting good performance. Chemical cleaning has a distinct advantage also. It does a much better job of removing organic (think oil) soils. It can also be used to apply a conversion coating, promoting adhesion and adding corrosion protection. Both methods have their place and both can work well on your furniture.

Now, to answer your question, what would I do with steel furniture? I would blast it, but there is an important proviso; you should use a two-coat process with one coat of zinc rich primer and then a good TGIC topcoat over that. If you use one layer of topcoat, I would go with zinc phosphate. Either method should work well. The zinc layer will protect the steel. I would not use iron phosphate and one coat of powder on steel for outdoor use. The initial adhesion will be good, but the corrosion protection is not good enough.

Find the follow up dialogue here.

Originally published in the August 2015 issue.